The present invention relates to multi-cell wireless radio telecommunication systems and, more particularly, to indoor cellular radio telecommunication systems and radio network planning.
Time Division Multiple Access (TDMA) radio communication systems, such as microcell cellular radio communication systems operating in accordance with the Global System for Mobile communication (GSM), for example, comprise a plurality of radio access units or radio base stations. Each providing service to a relatively small area or cell, generally having a radius of 10-100 m indoor and uptil 500 m outdoor.
The radio access units operatively connect to a central interface unit, also called Radio Exchange (RE) or Base Station Controller (BSC) which, in turn, connects to a Private Automatic Branch exchange (PABX), the Public Switched Telephone Network (PSTN) or the Integrated Service Digital Network (ISDN), for example. In GSM the BSCs connect through an intermediate Mobile service Switching Centre (MSC) to the PSTN, ISDN etc.
Between the radio access units and remote radio communication devices or subscriber units, such as mobile radio telephones of a TDMA radio communication system a duplex radio link can be established, generally occupying a pair of receive and transmit time slots grouped in a frame. A duplex radio link is established through a first time slot or transmission burst in a frame for downlink transmissions, i.e. from the radio access units to the remote radio communication devices, and a second time slot in the same frame for uplink transmissions, i.e. from the remote radio communication devices to the radio access units.
A GSM frame comprises 8 time slots, providing 4 full-rate radio links per carrier. In addition to these full-rate radio links, subrate transmission channels have been defined, the transmission capacity of which is a fraction of the transmission capacity of a full-rate time slot. Eight-rate traffic channels are specified and used for signalling, also called Stand-alone Dedicated Control CHannels (SDCCH).
GSM operates following a combination of Time and Frequency Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the Radio Frequency (RF) bandwidth into multiple carrier frequencies. In GSM, one or more carrier frequencies are assigned to each base station, and each of the carrier frequencies is divided in time using a TDMA scheme as disclosed above.
In practice, in particular at radio access units for indoor use, i.e. in cordless and microcell cellular (GSM) radio communication systems, very often not all the available time slots are used. In order to provide sufficient coverage for economic use of the time slots and the base station equipment involved, the RF transmission power of a radio access unit has to be increased to extend the coverage area or cell to service a larger number of radio communication devices. However, increased RF power creates interference in neighbouring cells, such that a strict frequency planning is required and, if applicable, measures have to be taken in order to reduce such interference.
International patent application WO 91/01073 discloses a cellular radio system wherein the cells are sub-divided in sectors, with each sector in a cell being served by one or more radio channels on a different carrier frequency or group of carrier frequencies to the rest of the sectors within the cell. The different sectors of the cell share time slots which can be assigned on demand to the sectors. With this dynamic assignment of common time slots, the communication capacity of a cell is more evenly spread over the sectors.
European patent application 0 444 841 likewise discloses a cellular radio system wherein time slots of a TDMA frame are fixedly or dynamically shared between adjacent sectors of a cell.
Although these known time slot sharing schemes enable balancing or evening of the net nominal system capacity in a cell and/or allocation of the available channel capacity to meet time-of-day capacity needs and the like, the cells and/or sectors are planned based on traditional RF carrier frequency re-use schemes.
It is an object of the present invention to provide a multicell TDMA radio communication system and equipment, in particular for but not limited to indoor use, providing a very efficient use of the available transmission capacity.
These and other objects and advantages are achieved by the present invention in a Time Division Multiple Access (TDMA) radio communication system, comprising radio access units or radio base stations arranged to provide duplex radio communication with remote radio communication devices in geographically limited areas or cells through a plurality of receive and transmit time slots grouped in frames.
In accordance with the invention, the radio communication system is arranged to provide radio communication in a cell through a limited subset of the plurality of receive and transmit time slots of a frame.
The system according to the invention limits the number of available receive and transmit time slots of a frame for communication in a particular cell, thereby providing for time slot planning instead of the well-known frequency planning widely used in GSM systems, for example.
With the invention time slots can be more effectively occupied while simultaneous interfering transmissions of radio links between cells can be prevented by an appropriate choice of the available time slots per cell. For example, in that the radiocommunication system is arranged to provide radio communication in neighbouring cells through different subsets of the plurality of receive and transmit time slots. Thereby effectively preventing interference between adjacent cells.
In accordance with a further embodiment of the invention, in a TDMA radio communication system having multiple RF carriers each of which comprising receive and transmit time slots grouped in frames, radio communication in a cell is restricted to a limited subset of the plurality of receive and transmit time slots of frames of different carriers. That is, in a cell several subsets of receive and transmit time slots can be provided, transmitted at different RF carriers.
The subsets of receive and transmit time slots available for radio communication in a cell can be adaptively selected or fixedly allocated.
In the adaptive selection embodiment of the invention, for communication in a cell, an arbitrary available subset of receive and transmit time slots is selected. Subsets of receive and transmit time slots of a particular frame already in use in neighbouring cells are prevented from selection. In practice, the occupied time slots of a frame during use in a cell may differ from time to time.
The radio communication devices can be designed for operation at the complete RF spectrum and time slots available for the radio communication system as a whole, such as with GSM, in order to provide handovers from one cell to another while the user of a radio communication device moves through the cells of the system.
In its simplest embodiment however, omitting the need for measurement and selection mechanism of time slots in the communication equipment, according to the invention, to each cell a limited subset of the plurality of receive and transmit time slots is predeterminedly allocated.
In a TDMA radio communication system operating with, say, eight receive and transmit time slots in a frame, time slots 0 and 1 can be akllocated to a first cell, time slots 2 and 3 can be allocated to a second adjacent cell, time slots 4 and 5 may be allocated to a third adjacent cell and time slots 6 and 7 can be predeterminedly allocated to a fourth neighbouring cell. Radio communication in a particular cell is only possible through the time slots allocated to such cell.
Those skilled in the art will appreciate that the latter embodiment of the invention can be easily controlled from the radio access units without requiring any control or selection mechanism in the radio communication devices.
In a minimum configuration, a subset of one pair of receive and transmit time slots of a frame is allocated to a cell. However, preferably, a subset of two pairs of receive and transmit time slots is allocated to a cell, in order to support inter and intra cell handovers.
As will be appreciated from the above, the radio communication system according to the invention can be operated in accordance with known TDMA protocols. However, the advantages of the present invention are best encountered if applied to a radio communication system operating following the GSM standard or equivalent.
A GSM radio communication terminal operating in a conventional GSM network can also operate in an (indoor) microcell cellular GSM system arranged in accordance with the present invention. The restriction of information exchange to a predetermined subset of time slots in a cell can be completely controlled through the radio access units of the system. Thereby preventing the need for so-called dual mode radio terminals, designed for outdoor operation in accordance with the GSM standard and indoor communication following a cordless radio communication standard, such as the Digital Enhanced Cordless Telecommunications (DECT) technology, for example.
In a further preferred embodiment of the radio communication system according to the invention, both the radio access units and the remote radio communication devices are arranged to operate at very low RF output power levels adapted for radio communication limited to pico cell or even nano cell ranges. That is cells having a radius of up to 10-15 meters. Contrary to the above-mentioned common approach for GSM systems, with the radio communication system according to the invention, in order to increase the communication capacity, the cell sizes are effectively decreased.
With the above low RF output power levels, it appears sufficient to allocate 2 or 3 RF carriers of a GSM radio system, for example, for indoor services throughout the GSM network as a whole.
In a preferred embodiment of the radio communication system according to the invention, a radio access unit comprises a wireless transmission interface operatively connecting to a wireless radio access controller and/or radio access exchange means. Preferably a wireless transmission interface operating in accordance with the DECT standard.
The invention further provides a radio access unit and radio communication device particularly designed for use in a TDMA radio communication system and/or a method of duplex radio communication according to the present invention.
The above-mentioned and other features and advantages of the invention will be best understood from the following description with reference to the enclosed drawings.